A fuel cell can be classified as a phosphoric acid type, a molten carbonate type, a solid oxide type, a polymer electrolyte type, or an alkaline type of fuel cell depending upon the kind of electrolyte used. Although each fuel cell effectively operates in accordance with the same basic principles, they may differ from one another in the kind of the fuel, operating temperature, catalyst, and electrolyte depending upon the type of cell.
Recently, polymer electrolyte membrane fuel cells (PEMFC) that have power characteristics which are superior to those of conventional fuel cells, as well as lower operating temperatures and quicker starting and response characteristics, have been developed. They are advantageous in that they can have a wide range of applications such as for a mobile power source for automobiles, a distributed power for houses and public buildings, and as a small electric source for electronic devices.
The polymer electrolyte membrane for a fuel cell should typically have high chemical and mechanical stability and high proton conductivity, and be made with a low cost.
A perfluoronated cation exchange membrane such as NAFION™ manufactured by the Dupont Company has been used as a conventional polymer electrolyte membrane. However, the polymer is very expensive and may permit the underisable cross-over of a liquid fuel such as methanol.
Therefore, for the polymer electrolyte membrane, a heat-resistant polymer such as polybenzimidazole (PBI) or polyethylenesulfone (PES) has been suggested.
However, while such heat-resistant polymers tend to have good chemical stability, they also tend to have low elasticity and hygroscopicity and thus, low proton conductivity.
European Patent No. 0 575 791 discloses a sulfonated aryl polymer such as polyetheretherketone (PEEK) and polyetherketone (PEK). However, such polymers can swell at a high temperature and thus a membrane made of the polymers is not suitable for a fuel cell.
There are approaches to making a polymer electrolyte membrane by blending a heat-resistance polymer and a sulfonated aryl polymer. However, such blended polymers tend to deteriorate membrane uniformity when hydrated by water while driving the fuel cell, and thus internal stress increases.